It is preferable to design an engine for a constant load so that a desired torque is generated at a low specific fuel consumption. However, it is difficult to design an engine for driving vehicles so as to have a low specific fuel consumption within the entire range of the engine operation, since load on the engine varies in a wide range.
FIG. 1 shows a fuel consumption characteristic of an engine for a vehicle at various specific fuel consumptions (g/ps.hr), in which the abscissa is engine speed (r.p.m.), and the ordinate is engine torque. Curve A shows running load (resistance) of a vehicle on a flat road. The curve A is determined by the air resistance of the body of the vehicle and the gear ratio of the transmission of the engine and the specific fuel consumption is determined by the performance of the engine. As seen from FIG. 1, the engine of FIG. 1 is so designed as to have lower fuel consumption at a higher engine torque range than that of a lower engine torque range and so that the curve A may pass through low fuel consumption zones in the higher engine torque range. However, the engine designed to have a low fuel consumption in a higher engine torque range has poor fuel consumption in a lower torque range.